TAMPA, Florida (Reuters) – Paul Ryan built his reputation as a fearless wonk who wasn’t afraid to put specific numbers on his small-government ideals. Now that he is the Republican Party’s vice presidential nominee, the devil lies in the details.

In a speech that marked his ascension onto the national stage, Ryan spelled out his conservative vision in the broad brush strokes of the presidential campaign, rather than the pointillistic data sets of the House of Representatives Budget Committee.

But the core message at the Republican National Convention was the same. Ryan said he and his boss, Republican presidential nominee Mitt Romney, must place the federal government on a crash diet and overhaul popular benefit programs in order to avoid a European-style debt crisis.

“The choice is whether to put hard limits on economic growth or hard limits on the size of government, and we choose to limit government,” Ryan said.

The take-no-prisoners stance has made Ryan a hero to conservatives, but it carries risks with a broader electorate.

While Americans may back the idea of spending cuts in the abstract, they tend to balk when presented with specifics. Polls show that more voters prefer keeping the Medicare health insurance plan for the elderly in place, rather than overhauling it as Ryan proposes.

“As rhetoric, it was an excellent speech in going over those broad principles. Likewise as rhetoric, it glossed over the hard realities of how you would achieve what he was talking about,” said Charles Franklin, a professor at Marquette Law School in Milwaukee, Wisconsin.

Perhaps it’s not surprising, then, that Ryan used personal stories to illustrate complex economic issues: the shuttered General Motors plant in his hometown of Janesville, Wisconsin, the small business his mother started at age 50, and the importance of Medicare to his mother, who smiled from the audience.

There were sins of omission. Ryan slammed Obama for ignoring a presidential debt panel, but failed to note that he himself served on the panel and voted against its findings. He also failed to mention that the GM plant closed before Obama took office.

Left unsaid were the tradeoffs Ryan and Romney would make in order to scale back the government to the level they envision.

“He didn’t say what the tough choices are,” said Steven Schier, a political science professor at Carleton College in Northfield, Minnesota. “You get into that in a convention speech, you lose the crowd, you lose the TV audience.”

As a vice presidential candidate, Ryan now must play second fiddle to a man who has often been reluctant to provide details of his own economic policies.

Romney has declined to say which tax loopholes he would close in order to lower income tax rates by 20 percent, and his own proposal for Medicare reforms lacks the specifics that would allow independent experts to determine how much they would cost taxpayers and beneficiaries.

Democrats, of course, are happy to fill in the blanks as they argue that Romney and Ryan would gut programs that benefit the middle class and the poor in order to cut taxes for the wealthy. With Ryan’s long voting record in Congress and several years of detailed budget proposals, they have plenty of material to work with.

Though Ryan is revered in Washington for his deep knowledge of fiscal policy, his skills as a salesman may be underappreciated.

Only eight of his fellow Republicans in the House of Representatives backed his plan to overhaul the Medicare prescription drug program when he introduced it in 2008. Within three years, nearly all of them supported it.

He has won re-election in his Democratic-leaning district by wide margins.

And he sounded like he was ready for his biggest sales job yet.

“Ladies and gentlemen, our nation needs this debate,” he said. “We want this debate. We will win this debate.”